Thermoplastic lining panels are used as chemically resistant, water-tight and gas-tight protection for concrete structures such as large containers, basins, ducts, etc. They are, therefore, subjected to considerable static and dynamic stresses such as movement of liquid, pressure changes, stresses derived from suppressed thermal expansion, etc. Owing to the stresses thus produced in the lining panels, mechanical affixing is necessary for producing a secure union between panels and concrete. This affixing must be such that the above-mentioned stresses can be dissipated by a uniform distribution over the entire surface of the panel into the concrete of the structure lined with the lining panels. This results in a field of forces of repressed stresses which is distributed uniformly over the entire area of the panel and avoids crack-propagating peaks of stress of the type occurring when the union between the panel and the concrete is unevenly distributed.
The magnitude of the stresses occurring in the lining panels also makes it virtually impossible to adhere the lining panels directly onto the concrete. Instead, the rear of the lining panels have to be provided in known manner (German Offenlegungsschriften Nos. 20 37 074 and 21 24 227) with a plurality of affixing knobs which are arranged in close proximity to each other in rows and which have a shape which is adapted to the structural conditions, preferably a circular or polygonal cross-section and undercut inclined running surface portions. The fixing knobs have to be provided in sufficient numbers per unit area to ensure that the stresses between the lining panels and the concrete are distributed uniformly.
Various methods of joining these fixing knobs to the lining panels are known.
In one method (German Offenlegungsschrift No. 21 24 227), large area lining panels composed of thermoplastic materials, which are particularly well suited to such lining purposes and are also inexpensive, are provided on their rear faces with the desired number of affixing knobs of a shape adapted to the structural conditions. To ensure that the joint between the relatively small end face of the knobs and the rear of the panel is able to take up the strong shearing and tensile stresses resulting from the above-mentioned stresses, the affixing knobs, in this method, are poured or injected onto a glass fiber mat which, in turn, has previously been affixed to the rear of the lining panel. The liquid polyester resin from which the affixing knobs are formed penetrates deep into the fabric of the glass fiber mat as the resin is being poured or injected and, after setting, forms with the mat a fracture-proof material which is firmly bonded to the rear of the lining panel.
The need to attach the glass fiber fabric beforehand and the application of optionally divided molds for the knobs to be poured or injected make this method relatively complex. It is also necessary to use three different materials, namely, the actual lining panel composed of thermoplastic, the glass fiber fabric for the glass fiber mat and the liquid plastic for the affixing knobs. Finally, the affixing knobs, which are formed from a liquid thermosetting resin and are no longer workable when heated, represent an obstacle in the hot working of the lining panels into round shapes and the formation of corners, if the affixing knobs are located in the corner region.
According to another known method (German Offenlegungsschrift No. 14 79 233), the above-described necessity of using three different materials can be avoided by producing the affixing elements from strips of material which are pared from the rear of a plastic panel using cutting tools and which are relatively broad in comparison with the thickness of the pared strip. The pared strips are bent around the root of the pared strip still connected to the panel during paring at a steep angle to the plane of the panel. Furthermore, parallel knicks and other knicks intersecting with them are provided. However, the resultant affixing elements are not suitable for conveying into the concrete the forces needed for suppressing stresses in the lining panel as, on the one hand, the root cross-sections of the pared strips are too small for this purpose and, on the other hand, the pared strips themselves are flexible over the height needed for practical fixing in the concrete and are not therefore resistant to bending. This design also hinders the uniform flow of the concrete needed on the surface of the panels and prevents the air from escaping and therefore bubble-free pouring of concrete against the lining panels.
According to another known method (French Pat. No. 11 02 294), lining panels can be produced from thermoplastic materials by extrusion affixing strips which are formed over the length of the rear of the panel and are optionally undercut can be provided and can be split into knob-like portions by the milling of intermediate spaces. It has been found that this method is suitable only for relatively thin thermoplastic panels of 2-3 mm, but not for the panels of 4-10 mm needed in the present method, as rejects are produced owing to the uneven flow of material during extrusion, and unavoidable hollows are formed on the side of the panel opposite the affixing strips. Another disadvantage of these extruded affixing strips lies in the fact that the forces are dissipated along parallel lines and over a relatively narrow striproot width, which obstructs uniform flow of stress inside the panel. If the strips are arranged in very close proximity to each other, there is a risk of the intermediate spaces not being filled by the conventional concrete containing coarse aggregates and of bubbles being formed. However, if the strips are arranged at a large distance from each other, the strips will bulge apart due to expansion, for example, under elevated thermal stresses, and this can lead to fatigue failure during a change of temperature.
It might also be possible to inject the lining panels in one piece together with the affixing knobs. However, the results of such a method are limited to the possible size of the injection mold and the established shape of the affixing knobs and to the distribution of these affixing knobs. Any change would necessitate the production of an expensive new mold. The abovementioned disadvantages (French Pat. No. 11 02 294) of the unavoidable and undesirable hollows due to the uneven distribution of mass on the surface of an extruded panel opposite the affixing knobs would also arise with an injection molded panel.